As an underwater vehicle travels through the water, there are many situations requiring that the relative attitude of the underwater vehicle be known as it relates to the bottom of the body of water. Similarly, as an aircraft flies over a land surface, there are many situations requiring that the relative attitude of the aircraft be known as it relates to the land surface. Currently, underwater vehicles and aircraft are equipped to make measurements of altitude, absolute pitch, and absolute roll. Each measurement is made, respectively, by an altimeter, a pitch sensor, and a roll sensor. In general, altimeters measure altitude by projecting a beam of energy down to the water's bottom or land surface, and then measuring the time it takes to receive the reflection. Conventional pitch and roll sensors measure absolute pitch and absolute roll, respectively, by using a device that compares the orientation of the gravitational field to a referenced vehicle orientation. However, none of these sensors account for slope in the water's bottom or land surface. Thus, the measurements from these conventional sensors cannot be used to estimate the attitude of an underwater vehicle/aircraft relative to an underlying bottom/land surface that is sloped.